Preparation of fluorine-containing compounds



Nov. 29, 1955 M. W. FARLOW ETAL PREPARATION OF FLUORINE-CONTAININGCOMPOUNDS Filed March 29, 1955 2 Sheets-Sheet 1 I3-\.. I ll |4 32PRESSURE q PUMP CONTROLER IO l f GASOMETER FRACTIONATI LIQUID N2MANOMETER-9 COLI'JMN TRAP I? I5 7 I FLOWMETEP MANOMETEP e 8 4 5 CYLINDERPRODUCT 1 191 CYLINDER FLEXIB INSULATING CONNECTION Fig. 2

CATHODE 2 GAS my.

OUT 5 ANOPE GRAPH|TE ELECTRODE INVENTORS MARK WENDELL FARLOW EARLLEONARD MUETTERTIES W1 MedAZA ATTORNEY Nov. 29, 1955 M. w. FARLOW ETAL2,725,410

PREPARATION OF FLUORINE-CONTAINING COMPOUNDS Filed March 29, 1955 2Sheets-SheetiE GAS INLET I COPPER CONDENSER GRAPHITE ELECTRODE INVENTORSMARK WENDELL FARLOW EARL LEONARD MUETTERTIES ATTORNEY LIQUID REACTANTUnited States Patent PREPARATION OF FLUORINE-CONTAINING COMPOUNDS MarkWendell Farlow', Wilmington, and Earl Leonard Muetterties, Hockessin,Delt, as'signors to E. I. du Pont de Nemours and Company, Wilmington,DeL, a corporation of Delaware Application March 29, 1955, Serial No.497,604 8 Claims; (CL 260653) of these fluorocarbons,tetrafluoro'ethylene, has achieved commercial success in the form of itspolymers; How-' ever, wider. uses for tetrafluoroethyIene and otherfluoro carbons would be attained were there more economical methods fortheir preparation.

The present invention is directed to a process for1 syn-" thesizingfluorocarbons, especially tetrafluoroethylene, which comprises heatingcarbon at a temperature of at Ieastl'SOO" C. with a compound composed ofcarbon; fluorine, and at least one other element, the atomic ratio offluorine to other element or elements being at" l'e'as't 1*:1, saidcompound containing no element other than hydrogen, carbon, fluorine,chlorine and bromine. A preferred group of these compounds having thesaid ratio of fluorine to other element or elements comprises thefinorochloroalkane's, fluorobroinoalkanes, fluoro'alkane's;fluorochloroalkenes, fluorobromoalken'es, and fluoroalke'nes'. c c Thereaction gives a mixture of products; the principal fluorocarbons beingtetrafluoro'ethylene and carbon tetrafl'uoride, with smaller amounts ofhexafluoroethane'. When the initial reactant is a fluorochloroorfluorobroino-co'mpound, substantial quantities of differentfluorochlorov or fiuoro'bromo-co'mpounds, respectively, are alsoprod-need. These latter can, of course, be re cycled to the reactionzone;

The process of this invention can be carried out'in various ways. In oneembodiment, the vaporiie'd 'comound or carbon. fluorine, anda'tleast-one other element, which the ratio of fluorine to the otherelement is at least 1-:1,-car'i be passed through atube made of carbonorof other refractorym'ate'ri'alpacked with carbonhea'ted at atemperature of at least 1500 C. Ther eactor' tube canibeheated' in anysuitable manner, e. g'., by an electricresistance or induction furnace.The gaseous} reaction products can then be passed through-'-c'oldcondensers, or traps, to' isolate li'qui'd' reaction products. Whentetia fluoroethylene is the} desired fluorocarbon, it is; preferable to:cool? the hot reaction products rapidly to a temperature below 400 C.The time of transition from the reaction temperature to 400 C. shouldnot exceed. one: second and isipreferably in the range of 0.0.01. to 01second tor-obtain best-yields, of tetrafluoroe'thylen'e. The liquefiedfluorocarbons and any unreacted: starting material can beisepae rated byfractional distillationin e'fiicient fra'ctionating columns. Optionally,the crude reactionproducts' can be passed through aqueous solutions ofan alkali, e. g'.,

2,725,410 Patented Nov. 29,

2 sodium hydroxide, to cool the reaction products and to abso'r anyacidic by-products. Moreover, it is sometimes desirable to employ anacceptor for freehalogen, such as sodium iodide.

A preferred embodiment of the invention involves passing a compound ofcarbon, fluorine, and at least one other element in which the ratio offluorine to the other ele: ment' is at least 1:1 through the areproduced by passing an electric current between carbon electrodes wherethe temperature is'genrally estimated to be in the range of 2500 to'4000* C. In this method, the reaction products can be cooled, purified,vand isolated as described previously. I p

Another embodiment of the process of invention which is particularlyuseful when the starting fluorineeontai'nin'gcarbon compound is liquidat ordinary temperatures consists iii submerging .a carbon aie iii' theliquid fluorine-containing carbon compound. In this par" ticular processthe fluorine=contairring reactant is v'ap r ized by' the arc, reactswith the carbon of the arc, and then very rapidly cooled by thesurrounding liquid reactant.

To obtainmost efl'ici'ent operation of this process, it is desirable torecycle any unreacted starting material and any undesired fluorocarbonprodiicts'. Thus,- to obtain highestyields of tetrafluor'oetiiylene fromthe reaction of carbon with a fluorine cofitainin'g compound such as,for example, dichlo'rodifluoromth-ane, any unreacftd 'dichlorodiflu'oromethane and the fluorocarbon by-prodiicts from the firstpass through the heated reaction zone are recycled. The fluorocarbonlay-products having one to two carbons" are turn converted tot'etrafluor'oethyled doting the recyclingstep by a process described inapplication Serial No. 477,678, filed December" 27, 1954; by W. Farlow,now U. S. Patent 2,709,192, and the fluorocarbon by-pr'odu'cts havingthree or more eariszs i atoms are converted totetrafluoroetliylene"during the recycling step by a process described inapplication Serial No; 390,461, filed November" 6, 1953, by Ml W.Farlo'w', now U. S: Patent 2,709;1'82.

The reaction: temperature must beat least 15 00 C: and can be highascan' practically be obtained by known m'ea'ns. Extremely hightemperatures; i. a, above 2500 6.,such as cart be achieved by the use ofacarbon arc, represent.- apreferred embodiment since they lead to higherconversions per pass and increase the proportions: of the highlyv'alu'able tetrafluoro'ethylene. in the reaction product. The relativeproportions of the two reactants are not critical insofar as the courseof the reaction is concerned.- However, it isdesirable for economicreasons to have the carbon present in excessin: orderl to? utilize tothe greatest extent the more expensive fluorine-contain ing carbon:compound.

The pressure at: which the reaction is carried? out canhe varied? overwide limits.- Satisfactory results' can be obtained at pressures"ranging from as low as 1 r'ntn. ofmercury absolute, or lower, up' tosuperatmospherio pressures.- Subatmospheric pressures are preferablewhen fluorohalo compounds are used at reactants"- si'nce low pressuresminimize the": reaction of free? halogen: with fluorocarbon products.-Atmospheric and superatrn'ose pheric pressures are useful whenit: is"desiredto' carry out the reaction with submerged electrodes:

The fluorine-containing carbon compounds? usedas reactant in the processof this invention ne'ed' notbe rigorously pure. Commercially availablematerials or compounds made By known methods are entirely" suit able.Anhydrous reactants are not essential; however, in is preferable to usereactants substantially fi e'e or moisture.

Any form of carbon, either amorphous" or crystalline;

is suitable. for use in theiprocess' of this inventidnb there can beused coal, graphite, diamond, charcoal, and the various forms of carbonblack, such as lamp black, acetylene black, bone black, etc. Thepowdered forms of carbon are, of course, used as packings in the form ofpellets or on supports, such as coke. In general, best results areobtained with active carbon, of which many well-known varieties areavailable commercially. In general, active carbon is very finely dividedporous carbon having a total surface area of at least 20 square metersper gram (Hassler, Active Carbon, Chemical Publishing Company, 1951,page 127). When a carbon arc is used in the process, the activity orstate of subdivision of the carbon is apparently of no consequence, butthe carbon must, of course, possess sufiicient conductivity.

The invention is described in greater detail in the following examples,which illustrate the reaction of the fluoride with the carbon in anelectric arc.

A flow sheet of the equipment and process used in reacting a compound ofcarbon, fluorine, and at least one other element with the electrodes ofa carbon arc is shown in Figure 1. A detail of an are used with gaseousreactants is shown in Figure 2. A detail of a type of apparatus having acarbon arc submerged in liquid fluorine-containing reactant is shown inFigure 3. Another form of apparatus which can be used is shown in Figure4.

Referring to Figure 1, the gas lines are of copper tubing. In a typicaloperation the compound of carbon, fluorine, and another element iscontained in cylinder or tank 1. Valves 2, 4, 15, and 19 are closed, andvalves 7 and 12 are opened. The apparatus is evacuated by means of pump13 to remove the air, trap 10 is cooled with liquid nitrogen, valve 7 isclosed, argon (or otherv inert gas) is admitted through valve 4 to thedesired operating pressure, and pressure controller 11 is set tomaintain that desired pressure. The are 6 is struck, the reactant gas ispassed through the are at the desired rate (flowmeter 3) and the productis condensed in trap 10, except noncondensable gases which pass throughcontroller 11, pump 13, and into gas reservoir 14. During operation theare inlet pressure (manometer will be appreciably higher than the exitpressure (manometer 9) because of the constriction involved in the arcpassages. When it is desired to stop the reaction, the arc current iscut off, valves 2 and 12 are closed, valve 7 is opened, cylinder 8 iscooled with liquid nitrogen, and trap 10 is allowed to warm to roomtemperature, and the volatile product is distilled into cylinder 8.Finally, if desired, cylinder 8 can be pumped to remove traces of argon,or other noncondensables, after which the cylinder valve 7 is closed andthe product is allowed to warm to room temperature.

In continuous operation where unreacted starting material andfluorocarbon by-products are recycled, the trap 10 is connected throughvalve to fractionating column 16 which separates, as far as possible,the tetrafluoroethylene from the reaction products of the arc, sends thetetrafiuoroethylene of greater or less purity, depending on the gradedesired, to tetrafiuoroethylene storage 18 and returns the remainingstarting material and fluorocarbon by-products through valve 19 andflowmeter 3 to the are 6. If desired, the by-products before being sentback into the arc 6 can be passed through an alkaline scrubber to removeacidic gases, such as carbon dioxide.

. A detail of an are suitable for use with gaseous reactants is shown inFigure 2. The electrodes consist of graphite cylinders. The waterjackets are made of electrically nonconductive material, or if they aremade of electrically conductive material they are insulated from theelectrode holders. The are is struck by contacting the two electrodesthrough manipulation of one of the two flexiblerubber connections, care.being taken to avoid contact with uninsulated portions of theapparatus.

Thereafter the electrode gap is controlled to etIect the requisitecurrent. Either direct or alternating current can be applied across theelectrodes at widely varying voltages. In the case of alternatingcurrent, the frequency of the current can be varied over a wide range.Good results are obtained when a direct current of 10 to 30 amperes at10 to 50 volts is employed. However, the process is not limited to theuse of this narrow range of amperage and voltage.

Referring to Figure 3, a detail is given of apparatus having a carbonarc submerged in a liquid reactant. In this type of apparatus thepyrolysis products are rapidly cooled to the temperature of therefluxing liquid reactant. In this apparatus the arc is operated in thesame manner as described in the preceding paragraph.

Referring to Figure 4, a detail is given of a type of carbon arcequipment which is very efiicient and which gives very high conversionsto fluorocarbons of the compounds of carbon, fluorine and at least oneother element as defined previously. In this equipment one electrode ishollow and the other is solid. The gaseous reactants enter the reactionchamber through holes in the upper electrode holder, pass around thesolid electrode and between the adjacent ends of the two electrodes andthen out through the center of the hollow electrode. When the solidelectrode is smaller in diameter than the inner diameter of the hollowelectrode, the arc can be operated with the end of the solid electrode ashort distance above the end of the hollow electrode, parallel with theend of the hollow electrode, or extending into the center of the hollowelectrode. The exact position selected is dependent on the particularreactant being passed through the arc, the position being chosen whichprovides the best arc under operating conditions. The relative positionsof the two electrodes can be changed, if desired, during operation. Thisis sometimes necessary to maintain the optimum arc. The solid electrodecan also be larger in diameter than the inner diameter of the hollowelectrode. If desired, it can even be larger than the outer diameter ofthe hollow electrode. The solid electrode is designated as the anode inthe drawing; however, it can be the cathode and the hollow electrode canbe the anode, if desired.

Example 1.-1,2-dichloro-1,1,2,2-tetrafluoroethane is passed through acarbon are produced by graphite electrodes of 0.1 inch internal diameter(Figures 1 and 2) at a rate of 63.6 grams per hour, an arc pressure of0.04 to 0.10 atmosphere, absolute. The arc is operated at 25 voltsdirect current and 18 amperes. The products are cooled from aretemperature to below 400 C. in less than about 1.0 second. The gaseousreaction product contains about 20 mole per cent tetrafiuoroethylene, 5mole per cent tetrafluoromethane, 30 mole per centmonochlorotrifluoromethane, 20 mole per cent dichlorodifiuoromethane,and 20 mole per cent of unchanged 1,2-dichloro-1,1,2,2-tetrafluoroethane.

Example 2.Monochloropentafluoroethane is passed through a carbon areunder the conditions given in Example 1 except that the flow rate isgrams per hour. The

gaseous reaction product contains about 15 mole per centtetrafiuoroethylene, 20 mole per cent tetrafluoromethane, 10 mole percent hexafluoroethane, 30 mole per cent monochlorotrifluoromethane, and20 mole per cent unchanged monochloropentafluoroethane.

Example 3.Dichlorodifluoromethane is p a s s e d through a carbon areunder the conditions given in Example 1 except that the flow rate is 3grams per hour and the reaction gases after quenching are passed throughgranular sodium iodide. The gaseous reaction product contains about 5mole per cent of tetrafiuoroethylene, 5 mole per cent carbontetrafluoride, 20 mole per cent monochlorotrifluoromethane, 5 mole percent 1,2-dichloro- 1,1,2,2-tetrafluoroethane, and about 65 mole per centof unchanged dichlorodifluoromethane.

ple 1 except that the flow rate is 30 grams per hour and the gaseousreaction product after quenching is passed through granular sodiumiodide. The gaseous reaction product contains about 5 mole per centtetrafluoroethylene, 25 mole per cent carbon tetrafluoride, 15 mole percent dichlorodifiuoromethane, and about 55 mole per cent of unchangedmonochlorotrifluoromethane.

The examples have illustrated this invention by the pyrolysis ofparticular fiuorinecontaining compounds in the presence of a carbon arc.However, the process can be carried out very satisfactorily in aninduction-heated graphite tube furnace such as that disclosed in thecopending application of M. W. Farlow, S. N. 477,678, filed December 27,1954, now U. S. Patent 2,709,192. This type of furnace is well suited tooperation at 1500" to 2500 C. The manner in Which the various elementsin the fluorine-containing carbon compound are joined is not critical.At the temperature at which the reaction is carried out, fluorocarbons,including tctrafluoroethylene, are formed. Consequently, a wide varietyof fluorinecontaining carbon compounds are operable. A group ofcompounds which can be employed satisfactorily is made up of those whichcontain at least one carbon atom and an insuflicient number of fluorineatoms per molecule to be a fluorocarbon, said number being, however, atleast twice the number of carbon atoms in the molecule. Specificexamples of compounds that can be used in the process of this inventioninclude: difluoromethane, trifiuoromethane, trifluorochloromethane,dichlorodifiuoromethane, dibromodifluoromethane, trifluorobromomethane(pentafluoroethyl)benzene, bis(trifluoromethyl)benzene,dichloromonofluorobis(trifluoromethyDbenzene,dichlorotetradecafiuoroheptane, monochloropentadecafluoroheptane,monochlorotrifluoroethylene, trifiuoroethylene, 1 ,2,2-trichloro-1,1,2-trifiuoroethane, 1,2-dibromoperfluoropropane, and1,2-dichlorohexafluorocyclobutane.

We claim:

1. A process for the preparation of fluorocarbons which comprisesheating carbon with a reactant composed of carbon, fluorine and at leastone other element, the atomic ratio of fluorine to such other element orelements being at least 1:1, and continuing said heating at atemperature of at least 1500 C. until a fluorocarbon is produced, andseparating the said fluorocarbon from the resulting mixture, saidreactant containing no element other than hydrogen, carbon, fluorine,chlorine, and bromine.

2. A process for the preparation of fluorocarbons which comprisesheating carbon with a reactant composed of carbon, fluorine andchlorine, the atomic ratio of fluorine to chlorine being at least 1:1,and continuing said heating at a temperature of at least 1500 C. until afluorocarbon is produced, and separating the said fluorocarbon from theresulting mixture.

3. Process of claim 2 wherein the temperature is at least 2500 C.

4. Process of claim 3 wherein the said reactant isdichlorodifluoromethane.

5. Process of claim 3 wherein the said reactant ismonochlorotrifluoromethane.

6. Process of claim 3 in which the said reactant is passed through acarbon arc at a temperature of 2500 to 4000 C.

7. Process of claim 3 wherein the said are employs a current of 10 to 30amperes at 10 to volts.

8. Process of claim 6 wherein said reactant is conducted through acarbon are at a pressure of 0.04 to 0.1 atmosphere, and the resultingproduct is cooled from are temperature to below 400 C. in less than onesecond, whereby a mixed product containing tetrafluoroethylene isproduced, and thereafter separating tetrafluoroethylene from theresulting mixture.

Simons et al.: I. A. C. 8., vol. 61 (1939), pages 2962- 2966.

1. A PROCESS FOR THE PREPARATION OF FLUOROCARBONS WHICH COMPRISESHEATING CARBON WITH A REACTANT COMPOSED OF CARBON, FLUORINE AND AT LEASTONE OTHER ELEMENT, THE ATOMIC RATIO OF FLUORINE TO SUCH OTHER ELEMENT ORELEMENTS BEING AT LEAST 1:1, AND CONTINUING SAID HEATING AT ATEMPERATURE OF AT LEAST 1500* C. UNTIL A FLUOROCARBON IS PRODUCED, ANDSEPARATING THE SAID FLUOROCARBON FROM THE RESULTING MIXTURE, SAIDREACTANT CONTAINING NO ELEMENT OTHER THAN HYDROGEN, CARBON, FLUORINE,CHLORINE, BROMINE.